1. Field of the Invention
The present invention relates to a production method of a drawn film, more particularly a production method capable of producing a drawn film or a porous film as a film with excellent evenness even from a resin material with a high melt viscosity and low melt drawing.
2. Description of the Related Art
Although being excellent in terms of strength, e.g., a high elastic modulus, a film made of a crystalline thermoplastic resin is easy to cause drawing spots and be deteriorated in the transparency. Therefore, techniques have been developed to solve such defective points; the formation of the drawing spots and the easiness of transparency decrease; by complicatedly controlling the drawing temperature, the drawing speed conditions in the case of biaxial drawing (for example, Japanese Unexamined Patent Publication No. 58-90924).
Nevertheless, even by doing so, the defects that the drawing spots are formed and the transparency is easy to be decreased can not reliably be solved, and furthermore, techniques to solve these disadvantageous points of the conventional techniques by strictly setting the drawing conditions have widely been known (for example, Japanese Unexamined Patent Publication No. 5-4276).
However, the above-described conventional production methods all require complicated control of drawing conditions, yet the uniformity of an obtained drawn film is insufficient, and especially in the case of a thermoplastic resin having a high crystallinity, accordingly, having a high elastic modulus and excellent strength, any reliable solution for the drawing unevenness has not been developed yet and there still remains room for improvement.
Also, as for porous films produced by conventional techniques, drawing unevenness has been found not a little and in the case of a film produced from a resin material with a high melt viscosity and a low melt drawing, for example, a thermoplastic resin composition containing a long molecular chain polyolefin resin, that is especially significant and the film is not sufficient yet in the uniformity and there still remains room for improvement.